What is a 250mm cap ?? its ona schematic for an amp I have but I cannot find anything on this cap except that it was used in an Echoplex. have not been able to find it on the net. Looks like a domino??
That's an old abbreviation for "micro-micro" farads (10-12). Today we call it a picofarad, or pf.
250pf is only 0.25nf, which is 0.00025µf - probably too small to do anything meaningful in most audio circuits today, but as any old analog tape technician can tell you, recorders of this nature had bias circuits. The recording heads were biased by a very high freq audio signal, usually on the order of 80KHz or better. That cap may have been in there as part of an oscillator circuit for creating the bias signal.
Can you link us to the schematic? Can you post an image of the cap?
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